An attempt will be made to establish unequivocally whether Balb/c3T3 cells respond with a stereotyped array of reactions to diverse external effectors such as chicken embryo cells have been shown to be. This will be done by studying in depth the uptake (phosphorylation) of uridine and inorganic orthophosphate, the rate of protein synthesis and the onset of DNA synthesis in 3T3 cells stimulated by simple inorganic complexes of calcium pyrophosphate. The degree of dependence and interdependence of each response will be determined. These experiments will determine whether mammalian cells, like chicken cells, respond coordinately and will indicate whether there is a single underlying modulator of the response in these cells. Since we have proposed that a major modulator of the metabolic response is availability of Mg 2 ion within the cell we shall study the effects of changing (Mg 2 ions)i on a relatively simple reaction, the uptake of uridine, which is limited by the rate of its phosphorylation. We shall compare the kinetic parameters of uridine uptake where (Mg 2 ion) in the medium is varied with those where (serum) is varied. We shall also extract uridine kinase, determine its Km for Mg-ATP and compare it with the apparent Km for Mg 2 ion of the uptake of uridine into intact cells and thereby obtain an estimate of free (Mg 2 ion) on those cells. We have found a close correspondence between the (Mg 2 ion) dependency of protein synthesis and the subsequent onset of DNA synthesis in the intact cell. We shall determine whether this relationship holds for extended time periods. We shall then determine whether the detailed changes in protein synthesis produced by varying (Mg 2 ion) are the same as those produced by varying (serum). Detailed studies will encompass degradation, polysome profiles and non-histone nuclear proteins.